A microstrip antenna is a small-sized light-weight planar antenna that uses a microstrip line formed on a dielectric substrate to transceive electromagnetic waves in a microwave band or a milliwave band, and used as, for example, a monitoring radar antenna.
FIG. 11 is a diagram illustrating a configuration example of a conventional microstrip antenna (e.g., Patent Literature 1). The microstrip antenna is a planar antenna in which on the front surface of a dielectric substrate 10, an antenna pattern 11 is formed, and on the back surface of the dielectric substrate 10, a grounding plate is formed. The antenna pattern 11 is configured to include a feeding point 12, a feed line 13, and radiating elements 14, and power is fed from the feeding point 12 to the radiating elements 14 through the feed line 13.
The directional characteristics of the microstrip antenna can be improved by providing the two or more radiating elements 14; however, when attempting to connect the two or more radiating elements to the common feeding point 12, it is necessary to provide a distributor S3 on the feed line 13. The distributor S3 has a T-shaped pattern adapted to branch a main feed line 21 into two or more sub-feed lines 22, and the radiating elements 14 are respectively connected to the branched sub-feed lines 22.
In the case of providing the distributor S3 on the feed line 13, a characteristic impedance mismatch causes power reflection at the distributor S3 to reduce an antenna gain. For this reason, in a conventional microstrip antenna, an impedance transformer 26 called a λ4 transformer is provided. The impedance transformer 26 is an element inserted between the main feed line 21 and the distributor S3, and has a line width different from that of the main feed line 21 and a line length of ¼ wavelength. By providing such an impedance transformer 26, the reflection at the distributor S3 can be reduced to suppress the reduction in antenna gain.
However, the above-described conventional microstrip antenna has a problem that the impedance transformer 26 limits a bandwidth to prevent the achievement of a wideband microstrip antenna. For example, in the case of a microstrip antenna used in the 60 GHz band, when using the impedance transformer 26, the bandwidth is limited to approximately 1 GHz, and therefore the microstrip antenna cannot be used for a bandwidth of several GHz or more.
Further, the T-shaped pattern forming the distributor S3 radiates undesired waves, causing the problem that the radiation of the undesired waves reduces radiation efficiency or adversely affects directional characteristics.